1. Field of the Invention
This invention relates to an air-fuel ratio control system which controls an amount of fuel supplied to an internal combustion engine, and more particularly to an air-fuel ratio control system which controls the supply of fuel injected into an intake pipe of the engine in a manner compensating for a fuel amount adhering to the inner surface of the intake pipe.
2. Prior Art
Conventionally, air-fuel ratio control systems for internal combustion engines are well known, which control operation of fuel injection valves arranged in an intake system of the engine in a feedback manner responsive to an output from an oxygen concentration sensor (air-fuel ratio sensor) arranged in an exhaust system of the engine such that the air-fuel ratio of a mixture supplied to the engine becomes equal to a desired value.
In air-fuel ratio control systems of this kind, a feedback gain which determines the control speed at which the air-fuel ratio is varied during the feedback control is determined by setting a frequency at which a correction value for correcting the air-fuel ratio is increased and decreased, in response to the rotational speed of the engine or by reducing the above control speed when the engine is in a decelerating condition, as proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-233333, or by varying the above control speed in response to the temperature of engine coolant, as proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-195349. Further, it has been proposed to determine the feedback gain in response to absolute pressure within the intake pipe.
As stated above, conventionally, the feedback gain is determined in response to engine operating parameters such as the engine rotational speed, the engine coolant temperature and the intake pipe absolute pressure.
On the other hand, in conventional internal combustion engines of the type that fuel is injected into an intake pipe, there is a problem that some of injected fuel adheres to the inner surface of the intake pipe, so that a required amount of fuel cannot be drawn into the combustion chamber, resulting in a difficulty of accurately controlling the air-fuel ratio of a mixture supplied to the engine. To solve this problem, there has been proposed a fuel supply control method which estimates a fuel amount which is to adhere to the inner surface of the intake pipe and one which is to be drawn into the combustion chamber by evaporation from the fuel adhering to the intake pipe, and determines a fuel injection amount based upon the estimated fuel amounts (Japanese Provisional Patent Publication (Kokai) No. 61-126337).
To improve this fuel supply control method, a supply fuel amount correction method has been proposed by the assignee of the present application, which calculates a fuel amount adhering to the inner surface of the intake pipe by the use of coefficients Ae and Be which are obtained by correcting a direct supply ratio (the ratio of a fuel amount drawn into the combustion chamber to the whole amount of fuel injected into the intake pipe) and a carry-off ratio (the ratio of a fuel amount carried off the inner surface of the intake pipe to the amount of fuel adhering to the inner surface), the two ratios being determined based upon the temperature of engine coolant and the intake pipe absolute pressure, and corrects the fuel amount to be supplied to the combustion chamber by the use of the amount of fuel adhering to the inner surface of the intake pipe and the coefficients Ae, Be U.S. Pat. No. 5,215,061.
However, according to 60-233333 in which the feedback gain is determined in response to engine operating parameters such as the engine rotational speed and intake air pressure, these engine operating parameters do not directly represent the amount of fuel actually drawn into the cylinders of the engine. As a result, the determined value of the feedback gain cannot always be optimal to operating conditions of the engine. There is the same disadvantage with 63-195349 using the engine coolant temperature for determining the feedback gain and the above-mentioned proposed method of using the intake pipe absolute pressure for determining the feedback gain.
Further, according to U.S. Pat. No. 5,215,061, an output from the oxygen concentration sensor is used to correct the fuel supply amount by the use of an amount of fuel adhering to the intake pipe inner surface in such a manner that a feedback correction coefficient KO2 is calculated in response to the output from the oxygen concentration sensor and a required fuel amount (required fuel injection amount) Tout is corrected by the calculated correction coefficient. As a result, the amount of correction of the Tout value can be excessive, resulting in degraded exhaust emission characteristics.